We analyze the polarization properties of polarization maintaining fiber and its basic measurement principle. Accord ing to the dependence of the phase difference between the two orthogonally polarized modes HF^x11 and HF^x11 on the wavelength of the injected laser,the wavelength-sweeping modulation method is adopted to measure the beat length of the fiber. The beat length and polarization extinction ratio of the PANDA polarization maintaining fibers are investigated in detail. Theoretical and experimental results are consistent with each other and both of them indicate that this method has the ad vantage of high precision and it is easy to operate .